Studies are being conducted to determine whether the two major metabolites of testosterone formed in the rat brain normally contribute to the activation of masculine sexual behavior by differentialy affecting neurotransmission in neurons which utilize 5-hydroxytryptamine (5-HT) or dopamine (DA) as neurotransmitters. A comparison will be made of the neural loci at which implantation of one metabolite, 5 alpha-dihydrotestosterone (DHT) or another metabolite, 17 beta-estradiol, most effectively facilitates mounting behavior in castrated rats treated systemically with the other steroid, and identify the neural sites where DHT most effectively inhibits estrogen-induced feminine sexual behavior. It will be attempted to relate the behavioral effects of estradiol and DHT to changes in the functional activity of 5-HT and DA neurons by studying the action of these steroids in gonadectomized rats treated with appropriate diets, neurotoxins, and monoaminergic agonists and antagonist drugs. Assessment will also be made of the effects of testosterone and its neural metabolites on the metabolism of 5-HT and DA in brain regions known to bind these steroids. Finally, studies will be carried out in attempting to explore the possibility that testosterone and its metabolites affect postsynaptic events associated with neurotransmission at 5-HT and DA synapses.